1. Field of the Invention
The present invention relates to a vibration member which uses a piezoelectric element, that is, an electro-mechanical energy conversion element as a driving source to form a driving vibration in an elastic member, a vibration wave driving apparatus which uses the vibration member as a driving source, and an apparatus provided with the vibration wave driving apparatus.
2. Related Background Art
As a vibration wave driving apparatus which uses, as a driving source, a vibration member for forming a driving vibration in an elastic member using a vibration source of a piezoelectric element as an electro-mechanical energy conversion element, there is a vibration wave motor as one system for relatively moving the vibration member and a contact member pressurized to contact the vibration member. The vibration wave motor serves as an actuator which can extract a large torque at a low speed, there is no cogging disposed on an electromagnetic motor, and the vibration wave motor is characterized by little rotation unevenness.
Particularly, in a traveling wave type vibration wave motor, a traveling vibration wave with a uniform vibration amplitude is generated in the elastic member, a moving member as a contact member pressurized to contact the elastic member is continuously driven, and therefore no rotation unevenness is generated in principle.
FIG. 21 is a perspective view of a vibration member of a conventional vibration wave motor.
Numeral 1 denotes an annular elastic member formed of a metal or the like, 2 denotes a piezoelectric element as an annular electro-mechanical energy conversion element, and the piezoelectric element 2 is bonded and fixed to one surface of the elastic member 1 by an adhesive.
For the piezoelectric element 2, electrodes are formed on both surfaces of a piezoelectric member constituted of ceramic subjected to a polarization treatment, voltage applying electrodes 3 are arranged at intervals in a peripheral direction on the surface shown in FIG. 21, and an entire surface electrode (not shown) for covering the entire surface is disposed on the surface bonded to the elastic member 1.
On the other hand, a wear-resistant layer of a wear-resistant friction material or the like is formed on the other surface (the surface opposite to the surface bonded to the piezoelectric element 2) of the elastic member 1, and a moving member (not shown) is pressurized to contact via the wear-resistant layer.
In FIG. 17, (1) to (3) are development diagrams showing a driving principle of a traveling wave type vibration wave motor.
In FIG. 17, (1) shows a first standing wave with a wavelength xcex excited in the vibration member (referred to as A phase), and (2) shows a second standing wave with a wavelength xcex excited in the vibration member (referred to as B phase). For the shown A and B phases, respective node positions (antinode positions) deviate from each other by a xc2xc wavelength. By simultaneously exciting and overlapping these two standing waves with a time phase difference of 90xc2x0, a traveling wave with a uniform amplitude can be synthesized as shown in (3) of FIG. 17.
For the vibration member in which the flexural traveling wave is excited in this manner, since a point apart from the neutral surface of flexural displacement performs elliptical movement, by pressing the moving member onto the top surface of the vibration member for contact in the vicinity of a vertex of elliptical movement, the moving member is driven by a friction force acting between the vibration member and the moving member.
For the piezoelectric element bonded to the elastic member constituting the vibration member for exciting the respective standing waves A, B, by forming a plurality of electrodes on a single disc by evaporation or the like and subjecting a plurality of areas to the polarization treatment, two standing waves deviating in phase from each other can be excited by a single piezoelectric element.
FIGS. 18A and 18B show a representative polarization pattern. Respective electrode groups of A and B phases are formed via a non-driving portion with a length of a xc2xc wavelength, and in each group, each electrode has a length of a xc2xd wavelength and the electrodes adjacent to each other are polarized in reverse directions as shown by symbols (+), (xe2x88x92) in FIG. 18A.
The respective electrode groups of A and B phases are short-circuited by means such as a conductive paste and a flexible printed board, a contraction and expansion force is therefore generated in a direction crossing at right angles to a polarization direction by applying a desired voltage between the electrode and a ground electrode on the back surface, and the aforementioned two standing waves are excited at the respective voltages of the A and B phases by applying a flexural moment to the vibration member.
However, in the aforementioned conventional example, when the polarization treatment is performed in order to form adjacent polarized areas in polarization directions opposite to each other in one piezoelectric element, the following problems arise.
FIG. 19 is a developed sectional view of the piezoelectric element of a portion in which the polarization directions in the adjacent polarized areas are opposite to each other. Arrows in FIG. 19 show electric force lines by differences of potentials applied to the respective electrodes during polarization.
As shown in FIG. 19, in a portion apart from a boundary (3) of two electrodes (1) and (2), the electric force lines run substantially in a thickness direction, and the polarization direction also runs along the direction of the electric force lines.
However, many of the electric force lines in the boundary (3) between the adjacent electrodes (1) and (2) run in a direction crossing at right angles to the thickness direction between the adjacent electrodes, instead of the thickness direction. Therefore, the present inventors have clarified that the polarization direction also runs in the direction crossing at right angles to the thickness direction of the piezoelectric element.
On the other hand, a flexural rigidity of the vibration member is determined mainly by the flexural rigidity of the elastic member and the rigidity of the bonded piezoelectric element.
Since the piezoelectric element is bonded to a position apart from the neutral surface of the vibration member in the thickness direction, the rigidity of the direction crossing at right angles to the vibration direction contributes to the flexural rigidity of the vibration member. For the piezoelectric element, a modulus of longitudinal elasticity is anisotropic depending on the applied polarization direction. Specifically, in FIG. 19, when the modulus of longitudinal elasticity of a direction parallel to the polarization direction of an area subjected to a treatment in the (ideal) polarization direction is Y11, and the modulus of longitudinal elasticity of a direction crossing at right angles to the ideal polarization direction is Y33, there is usually a relation of Y11 greater than Y33.
Since polarization is performed substantially in the thickness direction in the vicinity of the middle of the electrodes (1) and (2), the modulus of longitudinal elasticity of the thickness direction is Y11. However, when the adjacent electrodes are polarized in reverse directions, the modulus of longitudinal elasticity of the direction crossing at right angles to the thickness direction is Y33 in the boundary (3). Therefore, the modulus of longitudinal elasticity of the boundary of the electrodes polarized in the reverse directions indicates a smaller value than that of an electrode portion.
If there is partially a difference in the modulus of longitudinal elasticity of the elastic member, the following phenomenon occurs.
Specifically, for the standing wave excited by the elastic member, a propagation speed is determined by the flexural rigidity of each portion of the elastic member, and a line density. When the rigidity or the density is non-uniform, the propagation speed of flexural vibration partially changes, the wavelength of the excited standing wave changes, and wavelength unevenness occurs in some places.
FIG. 20 is a development diagram in which the wavelength unevenness generated by the polarization pattern of the piezoelectric element shown in FIG. 18 is shown centering on a driven portion with a xc2xc wavelength.
For the standing wave (wavelength xcex) in which the number of waves excited by the A phase electrode group is seven, ideally a vibration antinode corresponds to the middle of the electrode in the area of the A phase electrode group, and the vibration antinode coincides in position with the boundary of the electrodes in the area of the B phase electrode group.
Additionally, as shown in FIG. 19, since the modulus of longitudinal elasticity in the middle of the electrode (1) of the piezoelectric element is larger than the modulus of longitudinal elasticity of the boundary (3) of the electrodes (1) and (2), for the A phase standing wave, the flexural rigidity is high in the A phase electrode group area of the vibration member in which the antinode position of the standing wave is in the middle of the electrode of the piezoelectric element, and the flexural rigidity is low in the B phase electrode group area in which the antinode position is between the electrodes of the piezoelectric element.
Therefore, in the A phase area the vibration propagation speed increases and the wavelength is lengthened, while in the B phase area the vibration propagation speed decreases and the wavelength is therefore shortened.
Similarly, in the standing wave excited by the B phase electrode group, conversely, the wavelength is short in the A phase area and the wavelength is long in the B phase area.
As described above, since there is unevenness in the wavelength of each of the respective A and B standing waves each with the number of waves of seven, and there is deviation from a position phase xcex/4, it is seen from FIG. 20 that amplitude unevenness occurs in the amplitude of the synthesized traveling wave.
When there is unevenness in the traveling wave amplitude, unevenness occurs in the driving speed of the moving member. Therefore, there is unevenness in the press contact force of the moving member, or a contact surface is incompletely flat, and speed unevenness is therefore caused in the moving member by the relative position of the moving member and vibration member.
Moreover, the moving member slides in areas different in feeding speed at an equal speed, an area in which feeding forces are compensated each other is therefore produced, and efficiency is deteriorated by friction loss.
Furthermore, since the press contact force acting between the moving member and the vibration member differs with the position of the vibration member, deviation friction is caused on the friction surface of the vibration member, and the life of the motor is shortened as a result.
Moreover, in a conveying apparatus in which a powder material or another material small as compared with the wavelength is directly laid on the vibration member and conveyed, since the movement speeds of a plurality of waves are not averaged, the powder material gathers in a place where the traveling wave amplitude is small, or another problem arises, and smooth conveyance is inhibited.
The aforementioned amplitude unevenness not only depends on the polarization direction of the piezoelectric element but also occurs, for example, in the following situation.
Specifically, as the constitution of the vibration member, in addition to the single piezoelectric element, as shown in FIG. 22, a plurality of piezoelectric elements each having a length of xc2xd wavelength polarized in a single direction are bonded to the elastic member, or the length of the piezoelectric element is set to xc2xc wavelength.
In this case, since polarization is performed in the single direction in each piezoelectric element, the aforementioned problem of the difference in the modulus of elasticity by the polarization direction does not occur, but in a gap between the piezoelectric elements a sectional secondary moment is reduced as compared with other areas. Therefore, the aforementioned vibration amplitude unevenness is caused.
Even with the single piezoelectric element, as shown in FIG. 23, by applying the electrode on the entire surface, performing polarization to provide a uniform polarity, and polishing the electrode with a grindstone, a metal saw or the like to divide the electrode into a plurality of electrodes, a sectional shape has a partially cut groove 3-3, the rigidity between the electrodes is deteriorated, and the similar vibration amplitude unevenness is caused.
Moreover, even by dividing the electrode into a plurality of electrodes in the aforementioned method in an non-polarized state, bonding the piezoelectric element to the vibration member and subsequently performing the polarization treatment to vary the polarity, the similar result is obtained.
An object of the present invention is to provide a vibration member using an electro-mechanical energy conversion element as a vibration source in which wavelength unevenness generated in a plurality of standing waves is removed and a driving wave as a combined wave of the plurality of standing waves can be stabilized, a vibration wave driving apparatus using the vibration member as a driving source, and an apparatus provided with the vibration wave driving apparatus.
According to one aspect of the present invention, there is provided a vibration member, provided with an elastic member and an electro-mechanical energy conversion element, for combining a plurality of vibrations formed by applying an alternating signal to the conversion element and generating a driving vibration in the elastic member. In the vibration member, a partial ununiformity of rigidity of the vibration member caused by polarization of the conversion element is offset by partially changing the rigidity of the vibration member, so that a stable driving vibration of the vibration member can be outputted.
Other objects will be apparent by the following detailed description.